White LEDs are important in a wide variety of direct and indirect illumination applications. A white LED typically comprises a blue or ultraviolet LED whose emitted light is passed through a phosphor that converts the emitted light to white light. Such LEDs individually or in arrays can be used for room and space illumination, vehicle headlights, signs, and indicators. The emission spectra and color temperature of white LEDs is closer to sunlight than many forms of conventional lighting.
Unfortunately, it is difficult to efficiently make white LEDs enough to compete with conventional light sources. One source of inefficiency is coating the LED with the optimal thickness of phosphor. There should be sufficient phosphor to convert the emitted light but not so much as to block or absorb the white light output. Further details concerning the structure and operation of white LEDs may be found, for example, in U.S. Pat. Nos. 6,576,930; 6,608,332 and 6,614,179 which are incorporated herein by reference.
One approach to coating is to dispose the LEDs on a substrate, to dispense a thick transparent resin layer over the LEDs, and to apply over the transparent layer, a thin layer of resin containing phosphor particles. Ideally the resins dry to form a lens and an outer uniform phosphor layer around the diode. See, for example, U.S. Pat. No. 5,959,316.
There are, however, two problems with this approach. First, the uniformity of the phosphor coating is dependent on the shape of the transparent layer which, in turn, is difficult to control, especially when the resin is dispensed and dried over a wire bonded array. Second, the presence of the intervening transparent layer separates the LED from the phosphor, causing an undesirable broadening of the effective optical area.
In an attempt to overcome some of these problems, efforts were made to electrophoretically coat the LED with a uniform coating of phosphor. See U.S. Pat. No. 6,576,488. However this approach requires selectively masking some portions of the LED structure with conductive material (where phosphor is to be deposited) and other portions with dielectric material. The multiple masking and coating steps add substantially to the time and cost of fabrication.
Accordingly there remains a need for improved white LEDs and LED arrays and for an improved method of fabricating them.